This invention relates to a semiconductor current-conversion device, and more particularly to a semiconductor conversion device having semiconductor switching elements in a main circuit configured as a bridge circuit, which device has a drive-circuit configuration that minimizes device malfunctioning.
Among the known current-conversion devices for converting AC to DC and vice versa, semiconductor conversion devices having semiconductor switching elements in a main circuit configured as a bridge circuit have been used as inverters for electric power and other applications. For DC to AC conversion, these semiconductor conversion devices incorporate three-phase inverter circuits, an example of which is shown in FIG. 3, that convert direct currents to three-phase alternating currents with a variable frequency. As can be seen from FIG. 3, the three-phase inverter has a three-phase bridge circuit configuration and supplies three-phase alternating currents to a load, an electric motor 9, by converting a DC power supply V.sub.d. The three-phase inverter consists of an upper-arm circuit 10, which controls the higher electrical potential side of a three-phase alternating voltage, and a lower-arm circuit 30 which controls the lower electrical potential side of the three-phase alternating voltage.
The upper-arm circuit 10 consists of a main transistor circuit and a drive-circuit network. As shown in FIG. 3, the main transistor circuit of the upper-arm circuit 10 consists of bipolar transistors 20, 21 and 22 which act as switching elements and are positioned at U, V and W branches of the upper-arm circuit, respectively. The drive-circuit network of the upper-arm circuit 10 consists of component drive circuits 14, 15 and 16 which deliver driving signals to the respective transistors 20, 21 and 22, thereby causing these transistors to perform switching operations. The component drive circuits 14, 15 and 16 are powered by supplementary DC power supplies 17, 18 and 19, respectively, which are individually insulated from the outside. The source of control signals for the component drive circuits 14, 15 and 16 are not shown for sake of clarity.
The lower-arm circuit 30 shown in FIG. 3 has essentially the same configuration as the upper-arm circuit 10. A main transistor circuit of the lower-arm circuit 30 consists of bipolar transistors 40, 41 and 42 which act as switching elements and are positioned at X, Y and Z branches of the lower-arm circuit, respectively. A drive-circuit network of the lower-arm circuit 30 consists of component drive circuits 34, 35 and 36 which deliver driving signals to the respective transistors 40, 41 and 42, thereby causing these transistors to perform switching operations. The component drive circuits 34, 35 and 36 are powered by a common, external supplementary DC power supply 37.
The three-phase inverter represented by the circuit diagram shown in FIG. 3 is powered by a main DC power supply V.sub.d, which supplies the inverter with a DC current I.sub.c. The switching operations of the transistors 20-22 and 40-42 allow current flow through the load, the motor 9, and thereby allow supply of three-phase alternating voltage to the load.
In fabricating a three-phase inverter as shown in FIG. 3, two circuit substrates, examples of which are shown in FIG. 4, are typically used. With reference to FIG. 4, the two main transistor circuits consisting of transistors 20-22 and 40-42, respectively, are configured on a main-circuit substrate 3 by mounting a semiconductor chip 4 containing the two main transistor circuits. The two drive-circuit networks consisting of components drive circuits 14-16 and 34-36, respectively, are configured and integrally mounted on a drive-circuit substrate 5.
Both the main-circuit substrate 3 and the drive-circuit substrate 5 are housed inside a container consisting of a case 1 and a base 2 which has good heat conductivity. The main-circuit substrate 3 and the drive-circuit substrate 5 are connected to each other via an internal connection lead 6. Furthermore, an external connection lead 7 allows the drive-circuit substrate 5 to be supplied with external control signals and be connected to supplementary power supplies 17-19 and 37.
In the above-described conventional three-phase inverters, the drive-circuit substrate 5 is configured as shown in FIG. 5. With reference to FIG. 5, the drive-circuit substrate 5 is a double-sided printed wiring substrate, and the component drive circuits 14-16 and 34-36 are integrally mounted on the top surface 5a of the drive circuit substrate 5. In the individual component drive circuits 14-16, the ground electrode is tied to an emitter of the respective transistors 20-22 of the upper-arm circuit 10. Thus, the wiring patterns 11-13 for the component drive circuits 14-16, respectively, consist of a foil-like conductor formed on the top surface 5a of the drive-circuit substrate 5. The wiring patterns 11 and 12 for the respective component drive circuits 14 and 15 are integrally formed with a terminal portion tied to the connection leads 6 and 7. The wiring pattern 13 for the component drive circuit 16 has a separately formed sub-portion, a wiring pattern 51 formed on the bottom surface 5b of the drive-circuit substrate 5, which wiring pattern 51 extends to a terminal portion tied to the connection leads 6 and 7.
Next, in the individual component drive circuits 34-36, respective ground electrodes are connected to a line 45, to which emitters of the respective transistors 40-42 of the lower-arm circuit 30 are in turn connected, as shown in FIG. 6. The line 45 is a common ground-potential line having a common reference potential equal to the potential of the negative electrode side of the supplementary power supply 37. Thus, individual component drive circuits 34-36 are mounted on a common wiring pattern 31. The common wiring pattern 31 consists of a foil-like conductor on the top surface 5a of the double-sided printed wiring substrate 5. The wiring pattern 31 has a separately formed sub-portion, a wiring pattern 52 formed on the bottom surface 5b of the double-sided printed wiring substrate 5 serving as the drive-circuit substrate, which wiring pattern 52 extends to a terminal portion tied to connection leads 6 and 7.
The conventional drive-circuit substrate 5 shown in FIG. 5 which has wiring patterns on both of its surfaces 5a and 5b provides sufficient electric insulation between the individual component drive circuits 14-16, and also between the component drive circuits 14-16 and the wiring pattern 31 consisting of individual drive circuits 34-36.
In the three-phase inverter with the circuit configuration shown in FIG. 3, the upper-arm circuit transistors 20-22 and the lower-arm circuit transistors 40-42 alternately and periodically perform switching operations in response to external control signals in order to convert the main DC power supply V.sub.d to three-phase alternating currents. In this operation, the respective upper-arm circuit transistors 20-22 perform switching operations at time intervals corresponding to a phase angle of 120.degree., thereby keeping the three-phase alternating currents balanced. Furthermore, the two groups of the upper-arm circuit transistors 20-22 and the lower-arm circuit transistors 40-42 must alternately perform switching operations at time intervals corresponding to a phase angle of 180.degree. so that mutually opposing transistors, e.g., transistors 20 and 40, will not be simultaneously turned on.
One of the key considerations in configuring a semiconductor conversion device is the prevention of malfunctioning of semiconductor switching elements. A malfunctioning semiconductor switching element will cause a circuit arm to short, thereby resulting in excessive current flow through the circuit. Consequences are often serious, as destruction of the semiconductor switching element or the load equipment may occur.
The recent advances in semiconductor technology has substantially reduced the occurrences of semiconductor switching elements' malfunctioning, and has improved the reliability of semiconductor conversion devices in general. However, semiconductor conversion devices' malfunctioning may also be caused by factors other than the semiconductor switching elements, such as the configuration of the drive-circuit substrate. In a conventional three-phase inverter as shown in FIG. 3, the electrical potentials of the emitters of the transistors 20-22 vary at different times due to the above-mentioned switching operations of the transistors 20-22 and 40-42. Accordingly, in the drive circuit substrate of the conventional three-phase inverter, the electrical potentials of the respective wiring patterns connected to the transistors 20-22 are also constantly changing.
For the configuration of the drive-circuit substrate 5 provided in the prior art three-phase inverter as described above in conjunction with FIG. 5, the wiring pattern 13 on which the individual drive circuit 16 is mounted, and the wiring pattern 51 which is electrically connected to the wiring pattern 13, create an undesirable effect. The wiring configurations are such that the wiring patterns 13 and 52 mutually oppose each other with the double-sided printed wiring substrate 5 intervening between the two wiring patterns. Similarly, the pair of wiring patterns 51 and 12 mutually oppose each other. These opposing wiring patterns provide a relatively large parasitic capacitance because they have a configuration similar to that of a capacitor.
Since high-frequency currents easily pass through such a large parasitic capacitance, if the electrical potential of the emitter of the transistor 22 changes rapidly, high-frequency currents flow from the wiring patterns 13 and 51 to the wiring patterns 52 and 12, respectively, via the parasitic capacitor. High-frequency currents create electrical noise and may cause the transistors 21 and 40-42, which are driven respectively by the individual drive circuits 15 and 34-36 mounted on the wiring patterns 12 and 31, to malfunction.
It should be noted that the wiring pattern 31 is longer than wiring patterns 11-13 because the wiring pattern 31 has individual component drive circuits 34-36 integrally incorporated thereon. Furthermore, the wiring pattern 52 is longer than the wiring pattern 31 because the wiring pattern 52 must cover the entire range of the wiring pattern 31 and also include the terminal portion connected to the connection leads 6 and 7. As shown in FIG. 7, the wiring pattern 52 incorporates a sub-pattern 52a for the common ground-potential line 45 formed thereon. The wiring pattern 52a is formed at the periphery of the wiring pattern 52, as shown in FIG. 7.
The ground-potential line 45 provides a common reference potential for the individual drive circuits 34-36, and is electrically connected to the emitters of the transistors 40-42 and the negative electrode side of the supplementary power supply 37. Consequently, the ground-potential line 45 is electrically connected to the emitters of the respective transistors 40-42 and the main circuit line 101. In addition, part of the main DC electric current I.sub.c branches off from the main circuit line 101 and flows to the ground-potential line 45. In particular, if the main DC electric current I.sub.c contains high frequency components, the inductance of the main circuit line 101 will cause more high-frequency components to branch off to the ground-potential line 45.
FIG. 6, which illustrates the above-described effects in detail, shows a relevant portion of the ground-potential line 45 along with the surrounding circuit configurations. As illustrated in FIG. 6, when the transistor 41 is switched on, DC-side main electric current I.sub.CY, flowing to the negative electrode side of the main DC power supply V.sub.d, starts to flow through the main circuit line 101 to which the emitter of the transistor 41 is connected. Immediately after the transistor 41 is switched on, the value of the main DC current I.sub.CY increases rapidly when the current contains a large number of high-frequency components.
As an additional source of problems, there is always a parasitic inductance L1, either intended or unintended, associated with a portion of the main circuit line 101 located between the emitter of the transistor 41 and the emitter of the transistor 40. The existence of parasitic inductance L1 creates a diverted current I.sub.L1 to flow to the ground-potential line 45. As indicated in FIG. 6, the diverted current I.sub.L1 flows from the main circuit line 101, passes through a junction of the emitter of the transistor 41, through the ground-potential line 45, then through a junction of the emitter of the transistor 40, finally returning to the main circuit line 101.
The diverted current flow I.sub.L1 generates a high-frequency voltage at parasitic inductances L2, L3 and L4 associated with the ground-potential line 45. Consequently, circuit elements having a ground connected between the inductances, i.e., the circuit elements forming the individual component drive circuits 34 and 35, have varying ground potentials as a function of time. Changing ground potentials for each of the component drive circuits prevents proper functioning of the circuit elements. More particularly, the transistors 40 and 41 may malfunction due to the individual component drive circuits 34 and 35 delivering incorrect drive signals. Likewise, the same phenomenon also applies to the individual drive circuit 36 associated with the transistor 42.
There is therefore a need for improved wiring patterns for the drive-circuit substrates incorporated in semiconductor conversion devices such as inverters, which wiring pattern will minimize malfunctioning of semiconductor conversion devices.